Wall #1- 4" thick concrete block wall (all holes filled with mortar) and 1" plaster on both sides.. 6" total thickness.
The density of concrete is roughly 2400 kg/m3, so 4" (roughly 10cm) is going to have a surface density of around 240kg/m2.
Plaster is a bit lighter, around 1900 kg/m3, so 2" of that (roughly 5cm) would be around 95kg/m2.
So, average surface density of your wall will be around 335 kg/m2
Glass has a density of around 2500kg/m3, so if you really wanted to have the same surface density in your glass, you'd need glass a bit more than 5" thick! But that isn't necessary (unless you are aiming for extreme isolation)....
Wall #2- One 12mm layer drywall and One layer 6mm Fiber Cement Board.
Drywall density is around 685 kg/m3, so 12mm is roughly 8 kg/m2.
Fibercement density is around 1550 kg/m3, so 6mm is roughly 7 kg/m2.
So total average density is around 15 kg/m2.
To get the same surface density in glass, it would have to be 6mm thick.
So in theory, you would need one pane of glass 5" thick (127mm), and one pane of glass 1/4" thick (6.35mm).
However, that would be silly! This is one of those cases where "rule of thumb" theory doesn't jib with common sense reality. The problem is that you have one very high density leaf, and one very low density leaf, relatively speaking. Your inner-leaf wall has a surface density of only around 5% of the outer leaf, but the equations for MSM resonance assume that the masses are reasonably similar. In fact, optimum isolation is when both walls have approximately the same surface density. (m1=m2).
In the real world, it makes no sense to have glass 5" thick in your wall! Apart from anything else, the cost would be massively high, and I don't even know where you would buy such glass! It also makes no sense to have 6mm glass in a studio wall, as that is rather thin!
So the solution is to go for a reasonable but mathematically solid solution that will give you the isolation you need. You didn't say what that is, but assuming that you are facing the same flanking limits as everyone else, and taking into account that you don't seem to need high isolation anyway (your inner-leaf has low mass), let's assume that you are looking for around 45 - 50 dB of isolation, going down to about 40 Hz.
If you make your outer-leaf glass 19mm thick (3/4") and your inner-leaf 10mm thick (3/8"), and you have an air gap of at least 16cm (6") between them, then in theory you'd be getting close to 50 dB of isolation going down to roughly 40 Hz.
- Stuart -
